1. Field of the Invention
This invention relates to improvement in welding power supply, and more particularly to control of a power supply capable of achieving reductions of output current with a high response. More precisely, the invention contemplates to provide a method and device for controlling a power supply, useful for suppressing spatters in consumable electrode arc welding or for producing a weld of high quality in TIG welding.
2. Description of the Prior Art
In consumable electrode arc welding, there have been experienced various problems caused by spattering, e.g., a reduction of welding wire deposit efficiency, a drop of operational efficiency due to the necessity for removing scattered spatters, etc. Therefore, it has been a matter of great concern to suppress the spattering to a minimum. In order to analyze the factors which give rise to spattering, FIGS. 1(a) to 1(f) illustrate the sequential phases through which a molten droplet of a consumable electrode (hereinafter referred to as "welding wire") is transferred. In these figures, indicated at 1 is a welding wire, at 2 a workpiece, and at 3 an arc, showing: at (a) a phase immediately before short circuiting, in which an arc is generated across the gap; at (b) a phase in an initial part of short circuiting, in which a molten droplet comes into contact with a weld pool; at (c) a phase in a middle part of short circuiting, in which the molten droplet is securely in contact with the weld pool; at (d) a phase in the final part of short circuiting, in which "necking" occurs between the welding wire and the molten droplet as a result of transfer of the droplet to the weld pool; at (e) an instant of restriking an arc by the rupture of the short circuit; and at (f) a phase of re-arcing in which the distal end of the welding wire begins to fuse and a molten droplet grows. During a welding operation, the phases of FIGS. 1(a) to 1(f) take place repeatedly. Under these circumstances, spattering occurs at the moment of restriking an arc 3 by the rupture of the short circuit, namely, in the phase of FIG. 1(e), and, as well-known in the art, spattering occurs in a greater amount with a larger welding current at the moment of re-arcing.
In this connection, the power supplies which are generally employed in this sort of welding operations are DC power supplies of constant voltage characteristics, supplying welding current of the waveform as shown in FIG. 2 in which period T1 is a short circuit period (cf. FIGS. 1(b) to 1(d)) and period T2 is an arc period. As seen therefrom, the welding current is increased in the short circuit period T1 at a rate as determined by a time constant of an electric circuit for that period, and reduced in the arc period T2 at a rate as determined by a time constant for that period. Accordingly, the welding current of the conventional power supply reaches a peak at the moment of the rupture of short circuit or the moment or restriking an arc (FIG. 1(e)) when spattering takes place, thus performing a welding operation in a condition which is most susceptible to spattering. Therefore, a welding operation by the use of a conventional power supply suffers from a large amount of spattering, giving rise to various problems such as blocking of shielding gas flow by deposition of scattered spatters on the shielding gas nozzle, lowering the mechanical strength of the weld by entrainment of nitrogen or air into the molten metal.
In order to solve this problem, it has been proposed to use as a shielding gas a mixture of an inert gas (e.g., He, Ar or the like) and an active gas (e.g., CO or the like) thereby to prevent spattering. However, this method has been found to have no practical effect on the suppression of spattering unless either the welding current is greater than a certain value or the droplets are transferred in a state of a spray in free movement. On the other hand, there has thus far been employed a measure of lowering the welding current at the re-arcing time by suitably presetting the resistance and inductance of an electric circuit in welding operations to adjust the increasing and decreasing rates of the welding current in the short circuit period T1 and arcing period T2 of FIG. 2. However, the values of the resistance and inductance vary to a considerable degree depending upon the welding condition, so that it is extremely difficult to preset their values and this measure cannot be practically resorted to for the prevention of spattering.
Thus, the conventional spatter preventing means all fail to serve as a drastic remedy to the problem of spattering or to attain improvements in any substantial degree in this regard.